The MJO is characterized as a meso- α cluster of deep convection in the Indian Ocean that migrates eastward into the Indo-Pacific warm pool with a timescale between 30 and 90 days 7, 8.

SCSs are a perennial source of insured losses totaling $10B annually 5 in the US, and their extended-range prediction remains an area of significant stakeholder interest 6. The Madden–Julian Oscillation 1 (MJO) has recently been exploited as a source of skillful subseasonal predictability for United States (US) severe convective storms (SCSs tornadoes, severe hail, and damaging convective wind gusts) 2, 3, 4. The 53 MJO events that propagated past the Maritime continent revealed three prevailing MJO evolutions-each containing unique pathways and modulation of US tornado and hail frequency-advancing our knowledge and capability to anticipate these hazards at extended lead times. Statistically significant increases in US tornado and hail probabilities were documented 3–4 weeks following the period of the strongest upper-level divergence for the 53 active MJO events that propagated past the Maritime continent, contrasting with the 47 MJO events that experienced the barrier effect, during boreal spring 1979–2019.

MJO lifecycles were cataloged and clustered to document variability and potential pathways to enhanced subseasonal tornado and hail predictability. We approached this problem using pentad MJO indices and practically perfect severe weather hindcasts. However, the robustness of this relationship is unsettled, and the variability of physical pathways to modulation is poorly understood, despite the socioeconomic impacts that tornadoes and hail evoke. MJO phase has been correlated with anomalous tornado and severe hail frequency in the United States (US). The Madden–Julian Oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics and has a documented influence on extratropical extreme weather through modulation of synoptic atmospheric conditions.

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